1. Field of the Invention
The present invention relates to a polymer scale deposition preventive agent which prevents polymer scale deposition on the inner wall surface of a polymerization vessel and so on when a polymer is produced by polymerizing an ethylenic double bond-containing monomer in the polymerization vessel.
2. Description of the Prior Art
It is known that a polymer is deposited on the inner wall surface of a polymerization vessel and so on as a scale when an ethylenic double bond-containing monomer is polymerized.
The deposition of the polymer scale becomes more marked as the number of batches for polymerization increases, thereby reducing the yield of the polymer and the capability for cooling the inside of the polymerization vessel. If the polymer scale is peeled off from the inner wall surface of the polymerization vessel and is mixed in the polymer obtained, the quality of the product is lowered. Not only it takes excessive effort and time to remove the polymer scale deposited on the inner wall surface, but also there is a danger that the unreacted monomer remaining in the polymer scale causes damage to human bodies.
A method for preventing polymer scale from depositing on the inner wall surface of a polymerization vessel and so on when an ethylenic double bond-containing monomer is polymerized has been conventionally known in which a polymer scale deposition preventive agent is applied on the inner surface of the polymerization vessel, a stirrer, and so on to form a coating film. Known examples of this polymer scale deposition preventive agent include a mixture of a condensation reaction product of naphthols and an aldehyde compound and an inorganic colloid (and further a soluble polymer compound) (Patent Reference 1), a reaction product of hydroxymethanesulfinic acid sodium salt and naphthols (Patent References 2 to 5), and a mixture of the reaction product and a polyvinylalcohol (Patent Reference 6).
However, it has been revealed that, for example, a polymer scale deposition-preventive coating film formed of the polymer scale deposition preventive agent described in Patent Reference 1 has an inadequate polymer scale deposition-preventive effect near the gas-liquid interface in the polymerization vessel particularly in the case of a polymerization reaction with the polymerization temperature of about 60° C. or more. Furthermore, when the number of batches for polymerization increases to about 150 or more batches, polymer scales deposited near the gas-liquid interface begin to grow. When the degree of the growth becomes high, the polymer scales are finally peeled off and mixed in the polymer obtained, causing fish eyes in the polymer product.
As the number of batches repeated for polymerization increases, the layer of a polymer scale deposition preventive agent gradually becomes thick because the polymer scale deposition preventive agent is repeatedly applied to the inner wall surface of a polymerization vessel. This layer is partially pealed off and mixed in the polymer obtained, causing colored particles in the polymer product and discolorations such as yellowing of the molded product. This phenomenon is called initial discoloration.
[Patent Reference 1]
U.S. Pat. No. 5,576,370
[Patent Reference 2]
PCT international publication No. WO 96/35723
[Patent Reference 3]
PCT international publication No. WO 96/35724
[Patent Reference 4]
PCT international publication No. WO 96/39445
[Patent Reference 5]
PCT international publication No. WO 97/08210
[Patent Reference 6]
PCT international publication No. WO 96/39446